Catalyst for purifying exhaust gas, process for producing the same, and method for purifying exhaust gas

ABSTRACT

A catalyst for purifying an exhaust gas includes a support including rutile titania, an NO x  storage material including at least on element selected from alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support, and a noble metal loaded on the support. Since the rutile TiO 2  and the NO x  storage material form a fine composite oxide, the NO x  storage material is likely to decompose even when it is subject to the sulfur poisoning, and the NO x  storage material easily recovers the NO x  storage ability. Therefore, the NO x  storage material can be inhibited from the sulfur poisoning, and a high NO x  conversion ratio can be maintained even after the service at an elevated temperature.

TECHNICAL FIELD

The present invention relates to a catalyst for purifying an exhaust gas, a process for producing the same, and a method using the catalyst for purifying an exhaust gas, particularly, to a catalyst which can efficiently purify nitrogen oxides (NO_(x)) in an exhaust gas which contains oxygen excessively in an amount more than necessary for oxidizing carbon monoxide (CO) and hydrocarbons (HC) which are contained in the exhaust gas, a process for producing the same and a method for purifying the exhaust gas.

BACKGROUND ART

Conventionally, as a catalyst for purifying an automobile exhaust gas, a 3-way catalyst has been employed which carries out the oxidation of CO and HC and the reduction of NO_(x) simultaneously to purify an exhaust gas. With regard to such a catalyst, for example, a catalyst has been known widely in which a loading layer comprising γ-alumina is formed on a heat-resistant support, such as cordierite, and a noble metal, such as Pt, Pd and Rh, is loaded on the loading layer.

By the way, the purifying performance of such a catalyst for purifying an exhaust gas depends greatly on the air-fuel ratio (A/F) of an engine. For example, when the air-fuel ratio is large, namely on a lean side where the fuel concentration is lean, the oxygen amount in the exhaust gas increases so that the oxidation reactions of purifying CO and HC are active, on the other hand, the reduction reactions of purifying NO_(x) are inactive. Conversely, for example, when the air-fuel ratio is small, namely on a rich side where the fuel concentration is high, the oxygen amount in the exhaust gas decreases so that the oxidation reactions are inactive and the reduction reactions are active.

Whilst, in automobile driving, in the case of urban driving, the acceleration and deceleration are carried out frequently so that the air-fuel ratio varies frequently within the range of from adjacent to the stoichiometric point (ideal air-fuel ratio) to the rich state. In order to cope with the low fuel consumption requirement in such driving, a lean-side driving is needed in which a mixture containing oxygen as excessive as possible is supplied. Therefore, it is desired to develop a catalyst which can fully purify NO_(x) on the lean side as well.

Hence, an NO_(x)-storage and reduction type catalyst has been proposed in which an alkaline-earth metal and Pt are loaded on a porous support, such as alumina (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, etc.). In accordance with this catalyst, since the NO_(x) are absorbed in the alkaline-earth metal, serving as the NO_(x) storage material, and since they are reacted with a reducing gas, such as HC, and are purified, it is good in the purifying performance of NO_(x) even on the lean side.

In the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, it is believed that barium, for example, is loaded as the carbonate, and the like, on the support, and it reacts with NO_(x) to generate barium nitrate (Ba(NO₃)₂), thereby storing the NO_(x).

That is, in the above-described NO_(x)-storage and reduction type catalyst, by controlling the air-fuel ratio from the lean side to the stoichiometric point and to the rich side in a pulsating manner, the NO_(x) are stored in the NO_(x) storage material on the lean side. And, the stored NO_(x) are released at the stoichiometric point and on the rich side, are reacted with the reducing components, such as HC and CO, by the catalytic action of Pt, and are thereby purified. Therefore, since the emission of the NO_(x) is inhibited even on the lean side, a high NO_(x) purifying ability is exhibited as a whole.

In addition, it is understood that the purifying reaction of the NO_(x) in the NO_(x)-storage and reduction type catalyst comprises a first step of oxidizing NO in an exhaust gas to NO_(x), a second step of storing the NO_(x) on the NO_(x) storage material, and a third step of reducing NO_(x), which are emitted from the NO_(x) storage material, on the catalyst.

However, in the exhaust gas, SO₂ is contained which is generated by burning sulfur (S) contained in the fuel, it is further oxidized to SO_(x), such as SO₃, by the catalytic metal in an oxygen-rich atmosphere. Then, they are easily turned into sulfuric acid by the water vapor contained in the exhaust gas, and they are reacted with the barium, etc., to generate sulfites and sulfates, and it is understood that the NO_(x) storage material is thus poisoned and degraded. This phenomenon is referred to as sulfur poisoning. Moreover, the porous support, such as alumina, has a property that it is likely to absorb the SO_(x), and there is a problem in that the aforementioned sulfur poisoning is facilitated.

And, when the NO_(x) storage material is turned into the sulfites and the sulfates, it cannot store the NO_(x) any more, and, as a result, there is a drawback in the aforementioned catalyst in that the NO_(x) purifying ability decreases gradually in the service.

Moreover, since titania (TiO₂) does not absorb SO₂ it was thought of using a TiO₂ support, and an experiment was carried out. As a result, SO₂ was not absorbed by the TiO₂ and flowed downstream as it was, since only the SO₂, which contacted directly with the catalytic noble metal, was oxidized, it was revealed that the sulfur poisoning occurred to a lesser extent. However, when the TiO₂ support is employed, the initial activity was low, and it was revealed that there was a critical drawback in that the NO_(x) purifying performance was kept low after durability.

Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 6-327,945, it is proposed to use a support in which alumina is mixed with a composite oxide, such as a Ba—Ce composite oxide and a Ba—Ce—Nb composite oxide. In addition, in Japanese Unexamined Patent Publication (KOKAI) No. 8-99,034, it is proposed to use at least one composite support selected from the group consisting of TiO₂-Al₂O₃, ZrO₂—Al₂O₃ and SiO₂—Al₂O₃. By thus using the support in which the composite oxide is mixed, or by using the composite support, the NO_(x) storage material is inhibited from the sulfur poisoning, and the NO_(x) purifying ability after durability is improved.

However, since the recent increase of the high-speed driving, the improvement in the engine performance, and the regulation of the exhaust gas accompany the highly elevated exhaust-gas temperature, the exhaust-gas purifying catalyst is required to exhibit a further enhanced heat resistance.

The present invention has been developed in view of the aforementioned circumstances, and it is a primary object of the present invention to enable the NO_(x) storage material to be further inhibited from the sulfur poisoning, and to be capable of keeping a high NO_(x) conversion ratio even after the service at an elevated temperature.

DISCLOSURE OF INVENTION

A characteristic of a catalyst for purifying an exhaust gas according to the present invention, solving the aforementioned assignments, is that the catalyst, which is of an NO_(x) storage-and-reduction type, and disposed in an exhaust gas of an oxygen-rich atmosphere made by burning an air-fuel mixture whose air-fuel ratio, A/F (air/fuel), is 18 or more, so that NO_(x) in the exhaust gas is stored therein, and whose air-fuel ratio is perturbed from the stoichiometric point to a fuel-rich atmosphere periodically so that the NO_(x) stored therein is released therefrom, thereby carrying out reducing and purifying, comprises: a support including rutile type titania; an NO_(x) storage material including at least one element selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support; and a noble metal loaded on the support.

A characteristic of a process for manufacturing a catalyst for purifying an exhaust gas according to the present invention is that the process comprises the steps of: heat-treating by contacting a rutile type titania source with an NO_(x) storage material source including at least one element selected from the group consisting of alkali metals, alkaline-earth metal and rare-earth elements and by heat-treating them at 500-1,000° C., thereby forming a composite oxide powder of rutile type titania and an NO_(x) storage material; mixing the composite oxide powder and an alumina powder, thereby making a support powder; and loading a noble metal on the support powder.

Moreover, a characteristic of a method for purifying an exhaust gas according to the present invention is that a catalyst, comprising a support including rutile type titania, an NO_(x) storage material including at least one element selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support, and a noble metal loaded on the support, is disposed in an exhaust gas of an oxygen-rich atmosphere made by burning an air-fuel mixture whose air-fuel ratio, A/F (air/fuel), is 18 or more so that NO_(x) in the exhaust gas are stored in the NO_(x) storage material, and whose air-fuel ratio is perturbed from the stoichiometric point to a fuel-rich atmosphere periodically so that the NO_(x) stored in the NO_(x) storage material are released, thereby carrying out reducing and purifying.

BRIEF DESCRIPTION OF DRAWING

The FIGURE is a graph for showing the results of a Temperature Programed Reduction and elimination test on exhaust-gas purifying catalysts of Example No. 3 and Comparative Example No. 1, and illustrates the relationship between temperatures and amounts of eliminated sulfur.

BEST MODE FOR CARRYING OUT THE INVENTION

Titania (TiO₂) reacts with an NO_(x) storage material to form a composite oxide (e.g., BaTiO₃, etc.) partially at least. And, according to the study carried out by the inventors of the present invention, composite oxides generated by the reaction of anatase type TiO₂ and an NO_(x) storage material became coarse particles, however, it was revealed that particle diameters of generating composite oxides composed of TiO₂ and an NO_(x) storage material became extremely fine when rutile type TiO₂ was used.

And, when these composite oxides are used as a support, since the particle diameters are fine and the specific surface areas are large when the NO_(x) storage material in the composite oxides is subjected to the sulfur poisoning, the decompositions of the sulfates and the sulfites are facilitated even at a low temperature, it is believed that the NO_(x) storage material quickly recovers the NO_(x) storing function. Therefore, the catalyst is good in terms of the sulfur poisoning resistance, and can keep a high NO_(x) conversion ratio even after the service at an elevated temperature.

Whilst, in the anatase type TiO₂, the decomposition reactions of the sulfates and the sulfites are slow, and the decomposition reactions of the sulfates and the sulfites in a low temperature range are inferior to the rutile type. Moreover, the rutile type is better than the anatase type in terms of the dispersibility of a noble metal, the reason is not clear, however, it is revealed that the rutile type is less than the anatase type in terms of the sulfur poisoning extent. Therefore, in the present invention, the rutile type TiO₂ is used.

Besides, in a support in which an NO_(x) storage material is loaded on the rutile type TiO₂, there is a case where the composite oxides of the rutile type TiO₂ and the NO_(x) storage material do not exist initially. However, in the service as an exhaust-gas purifying catalyst or in a durability test, the composite oxides of the rutile type TiO₂ and the NO_(x) storage material are generated partially at least. Moreover, it is possible to include the composite oxides of the rutile type TiO₂ and the NO_(x) storage material, which are formed in advance, in the support. In this case, all of the support can be formed of the composite oxides of the rutile type TiO₂ and the NO_(x) storage material, or it is possible to make a support, a part of which contains the composite oxides of the rutile type TiO₂ and the NO_(x) storage material.

A particle diameter of the rutile type TiO₂ can preferably fall in the range of 15-100 nm. When the particle diameter of the rutile type TiO₂ is less than 15 nm, the particles of the composite oxides become coarse and the decomposition of the NO_(x) storage material, which is subjected to the sulfur poisoning, is hindered, because the whole particles react with the NO_(x) storage material. Moreover, when the particle diameter of the rutile type TiO₂ exceeds 100 nm, it is difficult to decompose the NO_(x) storage material, which is subjected to the sulfur poisoning, because the generating amount of the composite oxides composed of the rutile type TiO₂ and the NO_(x) storage material decreases. Therefore, when falling outside the aforementioned range, an NO_(x) conversion ratio decreases after the service at a high temperature in both of the cases.

The composite oxides of the rutile type titania and the NO_(x) storage material can be present in at least a part of the rutile type titania and the NO_(x) storage material, or the rutile type titania and the NO_(x) storage material can be made into the composite oxides as a whole.

It is possible to actively form the composite oxides of the rutile type TiO₂ and the NO_(x) storage material by contacting a rutile type TiO₂ source and an NO_(x) storage material source and heat-treating them at 500-1,000° C. When this temperature is less than 500° C., it is difficult to generate the composite oxides, and when it exceeds 1,000° C., the formed composite oxides grow granularly to decrease an NO_(x) conversion ratio. It is especially preferable to heat-treat them at 600-800° C. for 1-3 hours. In addition, the heat treatment can be carried out in the manufacturing of the catalyst, or can be carried out by a heat of an exhaust gas in the service as an exhaust-gas purifying catalyst.

As the rutile type TiO₂ source, the rutile type TiO₂ can be used as it is, or Ti compounds which are turned into the rutile type TiO₂ by the aforementioned heat treatment. Moreover, as the NO_(x) storage material source, it is possible to use compounds, such as acetates, nitrates and hydroxides, of at least one element selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements.

In addition, in the support of the exhaust-gas purifying catalyst according to the present invention, it is possible to include a porous substance of good gas-adsorbing ability, such as alumina, silica, zirconia and silica-alumina, and it is preferred that these porous substances are mixed with the rutile type TiO₂ to use. By means of this, the purifying performance is further improved.

Moreover, when the porous substance and the rutile type TiO₂ are mixed to make a support, it is preferred that the NO_(x) storage material is loaded on the rutile type TiO₂. By means of this, the generation of the composite oxides is made further easy, and the sulfur poisoning of the NO_(x) storage material is further inhibited.

As the NO_(x) storage material, at least one element is selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements, and, as the alkali metals, lithium, sodium, potassium, rubidium, cesium and francium can be listed. The alkaline-earth metals are referred to as the elements of group IIA in the periodic table of the elements, and barium, beryllium, magnesium, calcium and strontium can be listed. Moreover, as the rare-earth elements, scandium, yttrium, lanthanum, cerium, praseodymium and neodymium can be exemplified.

A ratio of the rutile type TiO₂ and the NO_(x) storage material can preferably fall in the range of the NO_(x) storage material/the TiO₂=1/9-7/3 by molar ratio, especially preferably fall in the range of 2/8-6/4 by molar ratio. When the amount of the NO_(x) storage material is smaller than this range, a sufficient NO_(x) purifying ability cannot be obtained, when the amount of the NO_(x) storage material is larger than this range, the purifying activity decreases because the NO_(x) storage material covers the surface of a loaded noble metal. In addition, there also arises a drawback in that the sintering of the noble metal is promoted.

As the noble metal, one of Pt, Rh and Pd, or a plurality of them can be used. The loading amount, in the case of Pt and Pd, can preferably be 0.1-20.0 g, especially preferably be 0.5-10.0 g, with respect to 120 g of the support. Moreover, in the case of Rh, it can preferably be 0.01-10 g, especially preferably be 0.05-5.0 g, with respect to 100 g of the support.

In manufacturing the exhaust-gas purifying catalyst according to the present invention, for example, it can be manufactured by mixing and heating the NO_(x) storage material and the rutile type TiO₂ to 500° C. or more to make at least a part of them into the composite oxides, by mixing the composite oxides with alumina, or the like, and thereafter by loading the noble metal.

In addition, it can be manufactured by mixing the rutile type TiO₂, on which the NO_(x) storage material is loaded, with alumina, or the like, on which the noble metal is loaded. In this case, it is possible to form the composite oxides by a heat of an exhaust gas in the service.

And, in accordance with the present invention, the exhaust-gas purifying catalyst is good in terms of the sulfur poisoning resistance, accordingly the sulfur poisoning of the NO_(x) storage material is inhibited in the service at an elevated temperature, by means of this, a high NO_(x) conversion ratio can be secured even after the service at a high temperature.

Moreover, in accordance with the present invention, the process for manufacturing a catalyst for purifying an exhaust gas can stably and securely manufacture the aforementioned exhaust-gas purifying catalyst.

And, in the method for purifying an exhaust gas according to the present invention, the exhaust-gas purifying catalyst according to the present invention contacts with an exhaust gas of an oxygen-rich lean atmosphere, the NO_(x) in the exhaust gas are thereby stored in the NO_(x) storage material, the exhaust-gas atmosphere is made into from the stoichiometric point to a rich atmosphere by periodically perturbing the air-fuel ratio from the stoichiometric point to a fuel-rich atmosphere, the NO_(x) stored in the NO_(x) storage material are thereby released, and the NO_(x) are reduced on the noble metal by the HC and CO in the exhaust gas.

By the way, in a lean atmosphere, the SO_(x) in the exhaust gas react with the NO_(x) storage material to generate the sulfates and the sulfites. However, the composite oxides are formed in at least a part of the NO_(x) storage material and the rutile type TiO₂, and the particle diameters are extremely fine. Therefore, the specific surface areas are so large that the decompositions of the sulfates and the sulfites are accelerated even at a low temperature, and the NO_(x) storage material quickly recovers the NO_(x) storage function. By means of this, the sulfur poisoning of the NO_(x) storage material is inhibited, and it is possible to stably purify the NO_(x) in the exhaust gas, emitted from a lean-burn engine, at a high conversion ratio.

EXAMPLES

Hereinafter, the present invention will be described concretely with reference to examples and comparative examples.

Example No. 1

A barium acetate aqueous solution was impregnated into a rutile type TiO₂ powder whose particle diameter was 35 nm, was dried, and was thereafter calcined in air at 650° C. for 3 hours. The molar ratio Ba/Ti of the resulting TiO₂ powder loaded with Ba was 1/9, when this powder was analyzed by the x-ray diffraction, the peaks of BaTiO₃ and the rutile type TiO₂ were observed. Namely, Ba was loaded so that it formed a composite oxide with the rutile type TiO₂.

Next, 100 g of the aforementioned TiO₂ powder loaded with Ba and 100 g of γ—Al₂O₃ were mixed uniformly with a ball mill, a dinitrodiamine platinate aqueous solution having a predetermined concentration was impregnated into the resulting powder in a predetermined amount, was dried, and was thereafter calcined in air at 300° C. for 1 hour. The loading amount of Pt was 2% by weight by metallic Pt conversion.

The resulting catalyst powder was formed by pressing, thereby obtaining pelletized catalysts having a size of 0.5-1.0 mm.

Example No. 2

Except that the molar ratio Ba/Ti was varied to 3/7, pelletized catalysts of Example No. 2 were prepared in the same manner as Example No. 1. Note that, when the TiO₂ powder loaded with Ba was analyzed by the x-ray diffraction, the peaks of BaTiO₃, the rutile type TiO₂ and BaCO₃ were observed.

Example No. 3

Except that the molar ratio Ba/Ti was varied to 5/5, pelletized catalysts of Example No. 3 were prepared in the same manner as Example No. 1. Note that, when the TiO₂ powder loaded with Ba was analyzed by the x-ray diffraction, the peaks of BaTiO₃, the rutile type TiO₂ and BaCO₃ were observed.

Example No. 4

Except that the molar ratio Ba/Ti was varied to 7/3, pelletized catalysts of Example No. 4 were prepared in the same manner as Example No. 1. Note that, when the TiO₂ powder loaded with Ba was analyzed by the x-ray diffraction, the peaks of BaTiO₃, the rutile type TiO₂ and BaCO₃ were observed.

Example No. 5

Except that a mixture aqueous solution of barium acetate and potassium acetate was used instead of the barium acetate aqueous solution, and that a molar ratio Ba/K/Ti was 4/1/5, pelletized catalysts of Example No. 5 were prepared in the same manner as Example No. 1. Note that, when the TiO₂ powder loaded with Ba/K was analyzed by the x-ray diffraction, the peaks of BaTiO₃, the rutile type TiO₂ and BaCO₃ were observed.

Example No. 6

100 g of a rutile type TiO₂ powder whose particle diameter was 35 nm and 100 g of γ—Al₂O₃ were mixed uniformly with a ball mill, a barium acetate aqueous solution having a predetermined concentration was impregnated into the resulting support powder in a predetermined amount, and the support powder was dried, and was thereafter calcined in air at 650° C. for 3 hours. The loading amount of Ba was 0.2 mol with respect to 100 g of the support. When the resulting powder loaded with Ba was analyzed by the x-ray diffraction, the peaks of BaTiO₃, the rutile type TiO₂ and BaCO₃ were observed.

A dinitrodiamine platinate aqueous solution having a predetermined concentration was impregnated into this powder loaded with Ba in a predetermined amount, was dried, and was thereafter calcined in air at 300° C. for 1 hour. The loading amount of Pt was 2% by weight by metallic Pt conversion.

The resulting catalyst powder was formed by pressing, thereby obtaining pelletized catalysts having a size of 0.5-1.0 mm.

Comparative Example No. 1

Except that an anatase type TiO₂ powder whose particle diameter was 18 nm was used instead of the rutile type TiO₂ powder whose particle diameter was 35 nm, and that the molar ratio Ba/Ti of the TiO₂ powder loaded with Ba was varied to 5/5, pelletized catalysts of Comparative Example No. 1 were prepared in the same manner as Example No. 1. Note that, when the TiO₂ powder loaded with Ba was analyzed by the x-ray diffraction, the peaks of BaTiO₃, the anatase type TiO₂ and BaCO₃ were observed.

Comparative Example No. 2

Except that an anatase type TiO₂ powder whose particle diameter was 18 nm was used instead of the rutile type TiO₂ powder whose particle diameter was 35 nm, pelletized catalysts of Comparative Example No. 2 were prepared in the same manner as Example No. 6. Note that, when the powder loaded with Ba was analyzed by the x-ray diffraction, the peaks of BaTiO₃, the anatase type TiO₂ and BaCO₃ were observed.

Test

Two conditions of model gases, which simulated automobile engine emission gases and whose air-fuel ratios were A/F=18 and A/F=14, were circulated through each of the aforementioned pelletized catalysts at an inlet temperature of 300° C. at intervals of 2 minutes repeatedly, and NO_(x) conversion ratios (initial NO_(x) conversion ratios) were measured, respectively, in this instance. The results are set forth in Table 1.

Moreover, a model gas, which was equivalent to A/F=18 and which included SO₂ in a concentration of 300 ppm, was circulated in each of the pelletized catalysts at 600° C. for 20 hours, and thereafter a model gas, which was equivalent to A/F=14, was circulated at 600° C. for 30 minutes, thereby carrying out a durability test, and thereafter NO_(x) conversion ratios (NO_(x) conversion ratios after durability test) were measured, respectively, in the same by manner as the initial NO_(x) conversion ratios. The results are set forth in Table 1.

TABLE 1 TiO₂ Ba/K/Ti NO_(x) C.R.*¹ (%) R.R.*² of P.D.*³ (nm) C.S.*⁴ P.D.*³ (nm) M.R.*⁵ Ini*⁶ A.D.*⁷ NO_(x) C.R.*¹ (%) of BaTiO3 Ex. #1 Rutile 35 1/0/9 60 54 90 16 Ex. #2 Rutile 35 3/0/7 78 64 82 20 Ex. #3 Rutile 35 5/0/5 75 66 88 19 Ex. #4 Rutiie 35 7/0/3 65 48 74 20 Ex. #5 Rutile 35 4/1/5 80 68 85 25 Comp. Anatase 18 5/0/5 74 45 61 37 Ex. #1 Ex. #6 Rutile 35 — 72 62 86 19 Comp. Anatase 18 — 70 56 80 30 Ex. #2 *¹denotes “NO_(x) Conversion Ratio”. *²denotes “Retention Ratio”. *³denotes “Particle Diameter”. *⁴denotes “Crystal Structure”. *⁵denotes “Molar Ratio”. *⁶denotes “Initial”. *⁷denotes “After Durability”.

Regarding the catalysts of Example No. 3 and Comparative Example No. 1, the dispersibility of Pt was investigated in both cases, initially and after the durability test, by the CO adsorption method. Moreover, by the microscopic observation, the particle diameter of loaded Pt was measured in both cases, initially and after the durability test. The results are set forth in Table 2.

TABLE 2 TiO₂ Pt D. ^(*1) (%) Pt P. D. ^(*2) (nm) Ex. #3 Ini ^(*3) Rutile 50 3 A.D. ^(*4) 13 13 Comp. Ini ^(*3) Anatase 15 10 Ex. #1 A.D. ^(*4) 7 22 ^(*1) denotes “Pt Dispersibility”. ^(*2) denotes “Pt Particle Diameter”. ^(*3) denotes “Initially”. ^(*4) denotes “After Durability”.

Further, concerning the catalysts of the examples and the comparative examples, the particle diameters of BaTiO₃ were measured by the x-ray diffraction, respectively, when they were in the initial states before they were subjected to the durability test. The results are also set forth in Table 1 collectively.

Furthermore, regarding the catalysts of Example No. 3 and Comparative Example No. 1, a durability test was carried out by circulating a model gas, which was equivalent to A/F=18 and which included sulfur in an amount of 50 ppm, at an inlet gas temperature of 550° C. for 2 hours. Then, the sulfur adhesion amounts were measured by a chemical analysis, and the results are set forth in Table 3.

TABLE 3 TiO₂ Sulfur Adhesion Amount (% by Weight) Ex. #3 Rutile 0.2 Comp. Ex. #1 Anatase 0.3

Moreover, concerning the catalysts of Example No. 3 and Comparative Example No. 1, in order to examine the elimination behavior of the adhered sulfur, the Temperature Programed Reduction was carried on the sulfur. The results are illustrated in the FIGURE.

It is understood from aforementioned Table 1 that the catalysts of Example Nos. 1 through 5 hold the retention ratios of the NO_(x) conversion ratios higher than the catalysts of Comparative Example No. 1, and that they exhibited the high NO_(x) purifying performances after the durability test. Moreover, it is the same when comparing Example No. 6 with Comparative Example No. 2.

On the other hand, it is appreciated from Table 3 that the adhering amount was less in the catalysts of Example No. 3, and it is seen from the FIGURE that the catalyst of Example No. 3 could release the sulfur from a low temperature of about 300° C.

Therefore, it is believed that the catalysts of examples exhibited the higher NO_(x) conversion ratios than the catalysts of comparative examples even after the durability test because they were inhibited from the sulfur poisoning more than the catalysts of comparative examples. Moreover, it is supposed that the sulfur dioxide poisoning was inhibited because the particle diameters of BaTiO₃ were so fine that the sulfates were likely to decompose. Therefore, it is understood from Table 1 that the particle diameter of BaTiO₃ can preferably fall in the range of 15-25 nm.

In addition, according to Table 2, loading Pt as fine particles and in a highly dispersed manner is one of the reasons that the catalysts of examples exhibited the excellent NO_(x) purifying performances. 

What is claimed is:
 1. A NO_(x) storage-and-reduction catalyst for purifying an exhaust gas that is disposed in an exhaust gas of an oxygen-rich atmosphere made by burning an air-fuel mixture whose air-fuel ratio, A/F (air/fuel), is 18 or more, so that NO_(x) in the exhaust gas is stored therein, and whose air-fuel ratio is perturbed from the stoichiometric point to a fuel-rich atmosphere periodically so that the NO_(x) stored therein is released therefrom, thereby carrying out reducing and purifying, said catalyst being characterized by comprising: a support including rutile titania; a NO_(x) storage material including at least one element selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support; and a noble metal loaded on the support, wherein at least a part of said rutile titania and said NO_(x) storage material forms a composite oxide.
 2. The catalyst for purifying an exhaust gas set forth in claim 1 characterized in that a particle diameter of said rutile titania is 15-100 nm.
 3. The catalyst for purifying an exhaust gas set forth in claim 1 characterized in that a particle diameter of said composite oxide is 15-25 nm.
 4. The catalyst for purifying an exhaust gas set forth in claim 1 characterized in that said NO_(x) storage material and said rutile titania are composed in a molar ratio of NO_(x) storage material/rutile titania=1/9-7/3.
 5. A process for manufacturing a catalyst for purifying an exhaust gas characterized by comprising the steps of: heat-treating by contacting a rutile titania source with a NO_(x) storage material source including at least one element selected from the group consisting of alkali metals, alkaline-earth metal and rare-earth elements and by heat-treating them at 500-1,000° C., thereby forming a composite oxide powder of rutile titania and an NO_(x) storage material; mixing the composite oxide powder and an alumina powder, thereby making a support powder; and loading a noble metal on the support powder.
 6. The process for producing a catalyst for purifying an exhaust gas set forth in claim 5 characterized in that said heat-treating step is carried out by heating at a temperature of 600-800° C. for 1-3 hours.
 7. A method for purifying an exhaust gas characterized in that a catalyst for purifying an exhaust gas, comprising a support including rutile titania, a NO_(x) storage material including at least one element selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support, and a noble metal loaded on the support, is disposed in an exhaust gas of an oxygen-rich atmosphere made by burning an air-fuel mixture whose air-fuel ratio, A/F (air/fuel), is 18 or more, so that NO_(x) in the exhaust gas are stored in the NO_(x) storage material, and whose air-fuel ratio is varied from the stoichiometric point to a fuel-rich atmosphere periodically so that the NO_(x) stored in the NO_(x) storage material are released, thereby carrying out reducing and purifying, wherein at least part of said rutile titania and said NO_(x) storage material forms a composite oxide.
 8. A catalyst for purifying an exhaust gas characterized by comprising: a support including rutile titania; a NO_(x) storage material including at least one element selected from the group consisting of alkali metals and alkaline-earth metals and loaded on the support; and a noble metal loaded on the support, wherein at least a part of said rutile titania and said NO_(x) storage material forms a composite oxide.
 9. The catalyst for purifying an exhaust gas set forth in claim 8 characterized in that a particle diameter of said rutile titania is 15-100 nm.
 10. The catalyst for purifying an exhaust gas set forth in claim 8 characterized in that a particle diameter of said composite oxide is 15-25 nm.
 11. The catalyst for purifying an exhaust gas set forth in claim 8 characterized in that a said NO_(x) storage material and said rutile titania are composed in a molar ratio of NO_(x) storage material/rutile titania=1/9-7/3.
 12. A process for manufacturing a catalyst for purifying an exhaust gas characterized by comprising the steps of: heat-treating by contacting a rutile titania source with a NO_(x) storage material source including at least one element selected from the group consisting of alkali metals and alkaline-earth metal and by heat-treating them at 500-1,000° C., thereby forming a composite oxide powder of rutile titania and an NO_(x) storage material; mixing the composite oxide powder and an alumina powder, thereby making a support powder; and loading a noble metal on the support powder.
 13. The process for producing a catalyst for purifying an exhaust gas set forth in claim 12 characterized in that said heat-treating step is carried out by heating at a temperature of 600-800° C. for 1-3 hours.
 14. A method for purifying an exhaust gas characterized in that a catalyst for purifying an exhaust gas, comprising a support including rutile titania, a NO_(x) storage material including at least one element selected from the group consisting of alkali metals and alkaline-earth metals and loaded on the support, and a noble metal loaded on the support, is disposed in an exhaust gas of an oxygen-rich atmosphere made by burning an air-fuel mixture whose air-fuel ratio, A/F (air/fuel), is 18 or more, so that NO_(x) in the exhaust gas are stored in the NO_(x) storage material, and whose air-fuel ratio is varied from the stoichiometric point to a fuel-rich atmosphere periodically so that the NO_(x) stored in the NO_(x) storage material are released, thereby carrying out reducing and purifying, wherein at least part of said rutile titania and said NO_(x) storage material forms a composite oxide. 